In a symmetric or private key encryption communication system, two devices in possession of a common secret or private key can perform both encryption and decryption using the secret key. A plaintext message may be encrypted using the secret key to produce encrypted plaintext or a cyphertext. The cyphertext may be decrypted using the secret key to re-produce the plaintext. The cyphertext may be accompanied by a signature. A receiver of the signature may process the signature to verify the authenticity of the sender.